I've used Evite for a long time, both as a recipient of invitations and as the originator. Initially being suspicious of the service as a potential spam outlet, I've used a special evite-specific email address for my main account. (Unfortunately, there's no control over which email address others use to invite you to events.) I've never received spam at that adddress, and remain confident that, as long as the company doesn't change hands, they are safe to use. Having used them once again for a recent event, I decided to revisit the issue and read through their privacy policy. A couple of sections are disappointingly unclear at first glance, but after careful examination, I'm convinced that Evite's policy backs up their claim to be "spam free." Backing the policy is a relatively big brand: Evite "is wholly owned by Ticketmaster and operated by the Citysearch.com division of Ticketmaster." Though some people might not trust Ticketmaster.
Looking for authoritative examples connecting Evite with poor practices or potential violations of their privacy policy: This November 2000 NYTimes article (on page 3) mentions Evite as an example of a company that puts tracking GIFs in HTML email, as part of an early lawsuit regarding privacy and disclosure on the web. A Usenet search comes up with a couple examples of Evite being used as a relay, which Evite actively prohibits, and which makes no use of Evite's customer list; plus one or two direct complaints with no details provided. A Google search for "evite spam" produces Evite's own site at the top (matching the phrase "Evite does not spam. Read our Privacy Policy to learn more"), plus dozens of other pages, all in Spanish-- because "evite" is Spanish for "avoid."
SpamAssassin is a popular spam flagging utility because it's extremely picky about what it considers "legitimate" email. SpamAssassin flags Evite mail as spam not because evite.com is on a spam blacklist, but because when I send you an evite, it originates from evite.com's mail servers but has my email address (at a non-evite.com server) in the From line. SA thinks this is suspicious and flags it, just to be safe.
Nevertheless, people still complain that Evite should be avoided, just because their service involves the mentioning of many email addresses and so could possibly be up to no good. Spam and the practice of spamming are so detested that simply telling someone that a company spams is enough to convince them beyond all repair, and the nature of an invitation service makes Evite automatically suspicious. SpamAssassin's flagging behavior doesn't help much, either. I'm compelled to jump to Evite's defense, if only because I'd like to be able to use them without getting yelled at by my invitees because they think I just signed them up permanently for a Make Porn Fa$t newsletter.
Beyond being able to trust certain commercial no-cost web services, I've often considered developing web services for private use. While Evite is a powerful and sophisticated implementation, an invitation service is a relatively trivial web/email application. If I built one and limited account creation to just me and my friends, we wouldn't have to worry about spam or annoying ads. It runs counter to the open spirit of the web (which I heartily endorse), but the resource demands of opening such a service to the public is why commercial services have had to bury users in ads and sell their email lists in the first place. I would probably open the software, however, so others could set up their own such services... Right after I finish my symphony...
I have used eVite almost since it launched and have never, to the best of knowledge, received a single bit of e-mail from them or through them that I didn't actually want. Despite their association with TicketMaster (which is a relatively recent development and makes me itch a bit), I have absolutely no qualms about using them and recommend them to friends.
Also, they've clearly made some performance tweaks to the system. The difference between my last large-scale invitation (for Break Bread with Brad in Austin) and the latest is amazing, a marked improvement in both the editing interface and the speed/responsiveness of the application.